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Bacterial Fossils

Studying ancient bacteria can be a little difficult. Despite having wonderfully complex internal biochemistry and fascinating ecological interactions they are essentially, when you get down to it, a little soggy bag. Little soggy bags do not preserve all that well; when bacteria die they just break apart, and are often eaten by other scavenging bacteria which see them as just free nutrients.

However some bacteria do fossilise, leaving behind perminant records of their existance. One of the ones that does it the best is cyanobacteria, which can form fossils in two ways. Firstly by forming little calcified shells around themselves as a product of increasing the dissolved carbon dioxide levels inside the cell:

Secondly cyanobacteria can group together with algae to form large layered mat-like structures called stromatolites. Slicing these very thinly reveals tiny cyanobacteria fossils, caught between the layers. There are stromatolytes in Australia that have been alive and growing (very slowly) for millions of years, despite looking faintly uninspiring.

Maybe not quite as impressive as dinosaurs...

While no other bacteria form fossils as nicely preserved as the cyanobacteria, they are capable of leaving behind visible remnants of their existence. The polypeptides of the bacterial cell wall (along with cytoplasm and some secreted lipids) can, under certain conditions, act as nucleation sites for minerals. This eventually leads to the organic cell being replaced by a little mineral cast of the bacteria. There is the possibility for quite a few artifacts with this (artifact being the scientific word for "result caused by the preservation process rather than the bacteria") the most common one being the creation of an artificial nucleus structure. As the bacteria degrades the cytoplasm tends to clump, and the crystallisation of minerals around a cytoplasm clot can in some cases create a structure that looks similar to the structure formed by a nucleus.

Endolithic bacteria that live in rocks can leave behind tiny canals in the rock surface that they bore into. These are a lot harder to find and interpretation is usually helped by the discovery of nearby alive endolithic bacteria. Bacteria have also been found trapped and mumified inside tree resin, a la the Jurassic park mosquito.

Once you start getting larger organisms the bacteria have a brand new niche to exploit. Fossilised bones sometimes show the results of a bacterial infection; while the bacteria themselves are not being preserved their presence is still seen in the fossil record.

Unfortunately although these glimpses are really interesting their also kind of frustrating from a biochemical point of view. They provide clues as to the lifestyles and processes within the bacteria but they are such tiny clues. Past biochemical processes are more often found inside the actual bacteria, by looking at clues in the genome and the genomes of related bacteria, than they are in the remains the bacteria leave behind.

4 comments:

Phoebe Cohen
said...

Bacteria may not be great at leaving body fossils, but they are wonderful at leaving other types of fossils - mainly biomarkers and isotopic signatures of their metabolic activity. Much of what we know about early earth's ecosystems comes from these two valuable proxies. We can't necessarily pin them down to a 'species' but we can get a lot of valuable information about their metabolisms, which goes a long way towards classifications.

@Phoebe: Thanks for the extra information! I've read about research done on carbon isotopic signature (to determine the start of photosynthesis) but hadn't looked into biomarkers.

@hhm: As far as I'm aware both the cyanobacteria and the algae in stromatolites are still alive. Most of the lower levels are mostly mineral deposits (and some fossils) but near the top there will be life, and growth.